Ol'ga Vollárová

Redox reactions of cobalt(III) complexes in aqueous organic mixtures effects: of single ion assumptions on initial state-transition state solvation analysis

SummarySolvent effects on the kinetics of redox reactions of cobalt(III) complexes were assessed in aqueous mixtures containing methanol,t-butanol, and acetone according to three different extra-thermodynamic assumptions —Wells, TATB, and TPTB** — at 298.2 K. The calculated values of transfer functions of individual ions and initial and transition states were often markedly different. In some cases these differences led to different conclusions in the evaluation of the role of solvation in the reactions investigated.

Effect of acid-base equilibria of reactants and the effect of waterorganic mixtures on the kinetics of oxidation of cysteinsulphenatobis(ethylenediamine)cobalt(III) ion by periodate

SummaryThe kinetics of oxidation of cysteinsulphenatobis(ethylenediamine)cobalt(III) ion by periodate were investigated in mixtures containing up to 40%t-BuOH andi-PrOH. The effect of acid-base equilibria on the rate constants and activation parameters ΔH≠, ΔS≠ and the effect ionic strength on the rate constant at different concentrations HClO4 are also reported. The solvent effects on the oxidation at the different pH are interpreted in terms of solvation of the initial and transition states.

Kinetics of oxidation of pentaammine-isothiocyanato-cobalt(III) ion with peroxodisulphate and periodate in water and water-organic mixtures

SummaryThe kinetics of oxidation of [Co(NH3)5(NCS)]2+ by peroxodisulphate and periodate were investigated at different ionic strengths, and at different temperatures and concentrations of perchloric acid. Rate constants and activation parameters are also reported in mixed water-organic solvents. It was shown that the acid-base equilibria involving S2O8−2, and IO4− ions are important in the reactions studied. Solvent effects on the oxidation are interpreted in terms of the solvation of both the initial state and the activated complex.

Solvent effects on the kinetics of oxidation of cysteinatobis(ethylenediamine)cobalt(III) ion by peroxodisulphate

The kinetics of oxidation of cysteinatobis(ethylenediamine)cobalt(III) ion, [Co(CysOS)(en)2]+, by peroxodisulphate were investigated at different ionic strengths, adjusted with various electrolytes, and at different temperatures and concentrations of perchloric acid. Rate constants and activation parameters are also reported for this oxidation in mixtures containing up to 40% acetone, t-butyl alcohol, isopropyl alcohol, or acetonitrile. The solubilities of [Co(CysOS)(en)2]ClO4 and KClO4 have been determined in water and aqueous acetone and aqueous t-butyl alcohol at 298.2 K. From these values, Gibbs free energies of transfer for [Co(CysOS)(en)2]+ and S2O82– have been estimated. The solvent effects on the oxidation have been divided into their initial-state and transition-state components.

Reactions of carbonyl compounds in basic solutions. Part 35

The kinetics of the alkaline hydrolysis and the configuration, conformation and electronic structure were studied using IR, NMR spectroscopy, X-ray analysis and AM1 theoretical calculations for a series of (Z)-4-(substituted benzylidene)-2-phenyl-4H-oxazol-5-ones (1) and compared with analogous results reported for (E)-4-benzylidene-2-phenyl-4H-furan-5-ones (5) and related compounds.

Solvent Effect on the Kinetics of Oxidation of [Co(en)2SCH2CH2 NH2]2+ by S2O8 2−

The effect of the solvent on rate of the oxidation of [Co(en)2SCH2CH2NH2]2+ by S2O82− in aqueous mixtures of methanol (MeOH), tert-butyl alcohol (ButOH), acetonitrile (AN), ethylene glycol (EG) and ethylene carbonate (EC) has been analysed as regards initial and transition state contributions. From kinetic measurements and solubilities of the reactants, the Gibbs transfer functions, ΔGt°, corresponding to transfer of reactants and activated complex from water to water-organic mixtures were evaluated. The trends of the transfer functions are discussed in terms of ion-solvent interactions and the structural effects in the solvent mixtures.

Transfer gibbs energies for ClO–3, BrO–3, IO–3, ClO–4 and IO–4 anions for water–acetonitrile and water–tert-butyl alcohol mixtures

The transfer Gibbs energies, ΔtrsG, of KClO3, KBrO3, KlO3, KClO4, KlO4 and the corresponding caesium salts have been obtained through the gravimetric measurement of solubility in aqueous mixtures with acetonitrile (AN) and tert-butyl alcohol (Bu tOH). Single-ion values of ΔtrsG have been calculated using the TATB assumption. The trends observed for ΔtrsG are discussed in terms of specific ion–solvent interactions and the structural effect of the solvent mixtures.

Activation volumes for peroxodisulphate oxidation of cobalt(III), iron(II) and nickel(II) complexes

SummaryDependences of rate constants on pressure (in aqueous solution up to 1.25 kbar) are reported for peroxodisulphate oxidation of hexacyanoferrate(II), tris(2,2′-bipyridyl)iron(II), tris(1,10-phenanthroline)iron(II), bis(1,4,7-triazacyclononane)nickel(II) and bis(1,2-ethanediamine)cysteinatocobalt(III) and its thioglycollato-analogue, and for periodate oxidation of the two last-named complexes. Derived activation volumes are reported and discussed in terms of intrinsic and solvational contributions. Rate laws and pressure effects on reactivity are reported for the reaction of peroxodisulphate with pentacyanoferrates(II) containing N-alkylpyrazinium ligands.

Kinetics of oxidation of bis(ethylenediamine)mercaptoacetatocobalt (III) ion by periodate in water-t-butanol and water-methanol mixtures at different perchloric acid concentrations

SummaryThe kinetics of oxidation of the bis(ethylenediamine)mercaptoacetatocobalt (III) ion by periodate were investigated as a function of perchloric acid concentration, ionic strength and temperature. Furthermore, the effect of a nonaqueous component of the solvent on the rate constant and activation parameters ΔH≠ and ΔS≠ in the water-t-butanol and water-methanol mixtures was estimated. The solvent effect is discussed from the viewpoint of the change in solvation of initial and transition states on going from water to a mixed solvent.

Studies on cobalt-tris(bipyridyl) complexes in acetonitrile–water mixtures

The Gibbs energies of transfer, ΔtG°, for [Co(bipy)3]3+ and [Co(bipy)3]2+ in the full range of acetonitrile–water mixtures were derived from solubility measurements of the picrates and partitioned into single-ion contributions based on the tetraphenylarsonium tetraphenylborate assumption. The enthalpies of solution were obtained from calorimetry of the perchlorates of [Co(bipy)3]3+ and [Co(bipy)3]2+. The single-ion transfer entropies were calculated ia the tetraphenylarsonium tetraphenylborate assumption. In addition polarography, cyclic voltammetry and UV–visible spectroscopy were also performed on these complexes in water–acetonitrile mixtures. Gibbs energies of transfer of [Co(bipy)3]+ were derived from the ΔtG° values for [Co(bipy)3]2+ and from electrochemical measurements. The TΔtS° values for [Co(bipy)3]3+ and[Co(bipy)3]2+ were calculated from the enthalpies and the Gibbs energies of transfer. The TΔtS° values of [Co(bipy)3]+ were obtained from the respective data for [Co(bipy)3]2+ and from the assumption of a negligible thermal diffusion potential. The Gibbs energies of transfer are accounted for in terms of hydrogen–hydrogen interactions between the methyl group of acetonitrile and the hydrogen atoms of the ligands of the studied complexes. The interpretations of TΔtS° values and the enthalpies of transfer were based on different regions for the structure of acetonitrile–water mixtures and on the possibility of the Co–bipy cations entering cavities in the water structure of water-rich mixtures.